Question

Which of these is not a function of the plasma membrane?

1) It is selectively permeable.
2) It prevents potassium ions from leaking out and sodium ions from crossing into the cell.
3) It acts as a site of cell-to-cell interaction and recognition.
4) It encloses the cell contents.

Answer 1

Option 2, which states that the plasma membrane prevents potassium ions from leaking out and sodium ions from crossing into the cell, is not entirely accurate. The plasma membrane's sodium-potassium pumps regulate ion movement, but do not prevent it outright.

Step-by-step explanation:

The question posed pertains to the functions of the plasma membrane, which is the biological membrane that separates the interior of all cells from the outside environment. The functions mentioned are:

1. It is selectively permeable.
2. It prevents potassium ions from leaking out and sodium ions from crossing into the cell.
3. It acts as a site of cell-to-cell interaction and recognition.
4. It encloses the cell contents.

While all of these are functions of the plasma membrane, the statement that the plasma membrane prevents potassium ions from leaking out and sodium ions from crossing into the cell (option 2) is not entirely accurate. The plasma membrane does have sodium-potassium pumps that actively transport potassium ions into the cell and sodium ions out of the cell, but the membrane itself doesn't strictly prevent their movement. This ion movement is regulated and requires energy, typically in the form of ATP.